Belt tensioner for safety belts for motor vehicles

ABSTRACT

A belt tensioner for safety belt arrangements in motor vehicles has a holder (11) on which are provided a belt winding reel (12) with a safety belt (13) coiled thereon, a blocking arrangement and an additional drive (15) connected via clutch (20) to the belt winding reel (12). The additional drive (15) contains a pressure source (16) which abruptly builds up a high pressure in a pressure chamber on being triggered and also contains a torque generating device (18) acted on by the pressure in the pressure chamber (17) and a drive shaft (19) connected to the latter. The drive shaft (19) is rotationally fixedly connectable to the belt winding reel (12) via the clutch (20). The torque generating device (18) includes a flexible draw band (21) which is secured at one end at a first mounting point (25) to the periphery of the drive shaft (19) or to the drive reel (22) concentrically mounted thereon and is partially wound onto the drive shaft (19), or onto the drive reel (22) and is secured at the other end, at a second mounting point (26) to the peripheral wall (30) of a band running chamber (23) within a housing (24). Pressure applied to the draw band causes belt tensioning movement of the drive shaft (19) or drive reel (22).

The invention relates to a belt tensioner for safety belt arrangementsin motor vehicles having a holder which is to be secured to the vehiclechassis and on the holder a belt winding reel with a safety belt woundthereon, a retraction spring arrangement by means of which the safetybelt is continuously and lightly so pretensioned in the retractiondirection that it only lies loosely on the vehicle occupant secured byit, a blocking arrangement, in particular between the belt winding reeland the holder of the vehicle chassis which blocks the safety beltagainst: being pulled out if suddenly pulled, and/or in the event of anacceleration resulting from an accident, and an additional driveconnected to the belt winding reel via a clutch, wherein the additionaldrive is normally separated drivewise from the belt winding reel by thedisengaged clutch, but is abruptly activated in the event of anacceleration resulting from an accident and is rotationally fixedlycoupled with the belt winding reel via the then engaged clutch in order,before the vehicle occupant falls into the safety belt, to wind up sucha length of the safety belt that the safety belt lies tightly on thevehicle occupant, with the additional drive (15) preferably having apressure source, in particular a pyrotechnical pressure source, whichcan be triggered or ignited by the accident dependent response of anacceleration sensor and which, on being triggered, abruptly builds up ahigh pressure in a pressure chamber, a torque generating device which isenergised by the pressure in the pressure chamber and a drive shaftconnected to the torque generating device which can be rotationallyfixedly connected with the belt winding reel via the clutch.

A torque generating device for belt tensioners is already known from theGerman laying open print 25 05 626 which is equipped with a drive wallwhich extends radially between the drive reel and the peripheral wall ofthe housing and which, together with a sealing wall which extendsradially inwardly from the housing to the drive reel, forms a pressurespace in which a drive pressure can be built up by a pyrotechnicalcharge, with the drive pressure acting on the drive wall which isconnectable with the drive reel and which thereby sets the drive reelrotating.

Problematic with this known torque generating device is the fact thatthe largest angle through which the drive reel can move through issomewhat smaller than 360° because the drive wall strikes against thesealing wall after somewhat less than one revolution of the drive reel.Moreover, the drive wall must be radially displaceably arranged becauseit must cooperate with a drive shaft which is expensive to manufactureand is provided with a toothed arrangement at its periphery. Themanufacture of the known torque generating device is thus complex andexpensive and, moreover, not sufficiently operationally reliable.

The dimensioning of the individual components should generally be suchthat a minimum tensioning length of the belt of 140 mm is achieved.Preferably a certain additional draw in length should however beavailable so that the belt can be drawn in or tensioned by the belttensioner, for example by a maximum of 205 mm. This is not possible withthe known belt tensioner.

After the tensioning the pressure which is generated by thepyrotechnical charge for only a short period of time generally drops offwhereupon the tensioning of the belt brought about by the vehicleoccupant falling into the belt triggers the blocking device and thevehicle occupant is reliably held by the now tensioned belt.

The object of the present invention is to provide a belt tensioner ofthe initially named kind in which the drive shaft or the drive reel, asa result of suitable dimensioning, can also be rotated through more than360° in the event of triggering of the pressure source as a result of anacceleration of the vehicle caused by an accident.

A further object of the invention is to provide a torque generatingdevice for a belt tensioner which can be manufactured with little costand complexity and which is nevertheless extremely reliable inoperation.

A further object of the invention is to provide a torque generatingdevice for a pelt tensioner which, in dependence on the angle ofrotation of the drive shaft, makes a desired change of the drive torquepossible., without the drive pressure being changed.

In order to satisfy this object the present invention provides that thetorque generating device has at least one flexible tension band which inparticular consists of spring steel and which is secured at one end at afirst mounting point to the periphery of the drive shaft, or to thedrive reel which is concentrically mounted thereon, and is partiallywound onto the drive shaft, or onto the drive reel, and is secured atthe other end at a second mounting point to the peripheral wall of aband running chamber which is arranged within a housing around the driveshaft, or around the drive reel and has a width corresponding to thewidth of the draw band in such a way that the draw band extends throughthe band running chamber while subdividing the band running chamber intotwo partial chambers sealed off relative to one another; and in that thepartial chamber which on being subjected to pressure brings about anunwinding of the tension band from the rotating drive shaft or drivereel is connected to the pressure space and with the other partial spaceis preferably vented; and in that, for the pressure-wise decoupling ofthe partial chambers, at least one sealing wall is provided whichextends between the housing and the periphery of the flexible draw bandwhich is wound onto the drive shaft, or onto the drive reelconcentrically mounted thereon.

In this manner the entire free-hanging length of the draw band extendingbetween the drive shaft of the drive reel and the peripheral wall isavailable for the action of pressure and thus for the generation offorce. By suitable choice of the diameter of the drive shaft or thedrive reel and the surrounding housing the reel or drive shaft can turnthrough a substantially larger angle than 360° on triggering of thetorque generating device.

In accordance with a first advantageous embodiment provision can be madethat the flexible draw band reverses its direction of curvature afterlifting away from the drive shaft or the drive reel and contacts theperipheral wall of the housing with this reversed direction ofcurvature.

It is however particularly preferred, when the flexible draw bandextends through the band running chamber up to its peripheral wall withthe same sense of curvature as it has as a result of being wound ontothe drive shaft or onto the drive reel.

This embodiment brings the advantage that the area of the flexible drawband which is acted on by the pressure from the pressure chamber issubstantially enlarged since the explosion pressure lifts the first bandlayer on the drive shaft or drive reel up to the sealing wall and thusacts on the draw band up to this point, so that with the same explosionpressure a substantially larger area of the draw band is acted on andthus the correspondingly larger tension force is transmitted to theflexible draw band. The wall surface enlargement corresponds, with onlyone partial chamber pair, essentially to the entire-drive shaft or drivereel periphery and with several partial chamber pairs essentially tothat part of the drive shaft or drive reel periphery which extendsbetween the adjoining sealing walls. Thus, practically the entire bandarea is available for the action of pressure from the position where theflexible draw band lifts off from the peripheral wall up to the sealinglip of the sealing wall. The invention can already be advantageouslyused with only a single flexible draw band.

A particularly preferred embodiment is characterised in that the sealingwall or walls has or have a sealing lip facing the outer surface of thecoiled up draw band and so extends or extend relative to the housing,and is or are so flexibly or hingedly designed or arranged, that thepressure in the associated pressure chamber sealingly presses thesealing lip against the outer surface of the coiled up tension band. Inthis way the sealing wall and in particular the sealing lip are pressedby the pressure in the associated pressure chamber against the outerperiphery of the draw band wound onto the drive shaft or drive reel andthus a pressure dependent, i.e. particularly effective pressure actionis produced. The greater the explosion pressure the better is the seal.

It is already known (EP 0 581 288 A1) that with such belt tensioners aclutch is arranged between the additional drive and the belt windingreel which is normally disengaged and which is first engaged directlyprior to the additional drive becoming active in the case of anacceleration brought about by an accident. A clutch of this kind is alsorepresented by the radially displaceable drive wall of theabovementioned DE-OS 25 05 626.

A further object of the invention is to make available a particularlyadvantageous design of a clutch of this kind which can be used inparticular in connection with the torque generating device of thepresent invention which has one or more draw bands.

In order to satisfy this object the invention provides that the clutchcan be engaged by the pressure generated by the triggering of thepressure source and the additional drive is separated from the pressuresource by a valve arrangement until the clutch has engaged.

For this arrangement it is particularly advantageous if the clutch has afirst clutch part, in particular a clutch disk which is rotationallyfixedly connected with the additional drive or with the torquegenerating device, and a second clutch part, in particular acounter-disk which is rotationally fixedly connected to the belt windingreel and is preferably formed by the one end guide disk of the beltwinding reel. In this arrangement provision is in particular made thatthe first coupling part is at least partially axially displaceable inorder to be able to engage with the counter-disk; and that the firstclutch part can be brought into engagement with the second clutch partby a spring force but is held against the spring force until triggered.

It is particularly preferred for the first clutch part to be a clutchdisk which is biased by a spring force towards the counter-disk; for theclutch disk to be preferably held at its outer periphery by a bayonetclosure connection in a disengaged position against spring force; andfor the bayonet closure to be releasable by relative rotation of theclutch housing and of the clutch disk. In particular it is furthermoreexpedient if the coupling disk is itself of resilient design andconsists in particular of spring sheet steel and is stressed in thenormal state between its center and its outer periphery to anessentially conical shape in such a way that the clutch is disengagedand if, on cancelling the stressing, for example through the release ofthe bayonet closure, the coupling disk is returned to its normalposition, in particular to its planar normal position in particular as aresult of rotation of the bayonet connection, and thereby enables theengagement for the clutch to engage.

A particularly preferred embodiment provides that the clutch has aslider which is displaceable by the pressure of the pressure source andwhich brings about the engagement of the clutch on displacement by thepressure of the pressure source.

In particular provision should be made that the slider has a pistonregion acted on by the pressure of the pressure source and, at the sideremote from the pressure actuation, a preferably thinner actuating rodwhich cooperates via an abutment projecting laterally sideways from itwith a counter-abutment provided at the periphery of the coupling diskin such a way that on advancing the actuating rod in the case ofpressure loading of the piston region the clutch disk is turned throughsuch an angle that it is rotated out of the disengaged blocking positioninto a position in which the spring force can bring the clutch disk intoengagement with the counter-disk, in particular with the guide disk ofthe belt winding reel.

It is of particular advantage when, in a first stage of the rotarymovement of the clutch disk brought about by the slider, the holdingnoses released from the counter-noses on the clutch housing at theperiphery of the clutch disk and, in a second stage of the rotarymovement--in so far as the engagement noses have, in the first stage,not yet entered into engagement with the engagement cut-outs--the clutchdisk is turned further by the advanced slider until the engagementcut-outs of the clutch disk have come axially into alignment with theengagement noses of the counter-disk, whereupon the resilient clutchdisk snaps over the engagement noses.

The concept of the invention of this further embodiment is thus to beseen in that, by suitable guidance of the pressure gases which are madeavailable from a pressure source, these can first be used to engage theclutch and only subsequently to actuate the additional drive.

It is important that on rotation of the clutch disk for the engagementof the clutch relative to the drive shaft, or to the drive reel of thetorque generating device, an adequate clearance is available so that theclutch disk can rotate without transferring a torque to the drive shaftor to the drive reel, until the engagement of the clutch has takenplace.

In order to ensure this, provision is made, in accordance with theinvention, that a peripheral clearance is present between the driveshaft, or the drive reel, and the clutch disk, such that, on rotation ofthe clutch disk contrary to the belt retraction direction, the bayonetclosure is first opened and then a further rotation of the clutch diskis possible until the engagement cut-outs snap over the engagementnoses, in so far as this alignment was not already present on opening ofthe bayonet closure.

The invention will now be described in the following by way of exampleonly and with reference to the drawings in which are shown:

FIG. 1 a first embodiment of a belt tensioner which operates with anignitable pyrotechnical charge as seen in a perspective explodedillustration,

FIG. 2 a somewhat enlarged perspective illustration of the sameembodiment of a belt tensioner in the assembled state as seen inperspective view from the opposite side to that in FIG. 1,

FIG. 3 a perspective view of the same article seen more from the side ofthe additional drive in which, however, for the purpose of illustration,the one end face cover of the band running space and the retractionspring arrangement have been omitted, with the view being shown in theinitial position directly after the start of ignition of thepyrotechnical charge,

FIG. 4 a corresponding view to that of FIG. 3 but during a belttensioning process which is already underway,

FIG. 5 an end view of a further preferred embodiment of a housing of abelt tensioner containing a band running space with the end coverremoved,

FIG. 5a an enlarged section from FIG. 5 with another embodiment of thereleasable mounting of the sealing walls to the housing,

FIG. 6 a section on the line VI--VI in FIG. 5,

FIG. 7 an enlarged end view of a section of a preferred embodiment ofthe drive reel of a belt tensioner in accordance with the invention inthe region of the draw band mounting point,

FIG. 8 an enlarged section of the end region of a draw band providedwith a spring tongue for mounting in a peripheral recess of the drivereel of FIG. 7,

FIG. 9 a perspective view of the belt tensioner of the inventionanalogous to FIGS. 3 and 4 but with the additional drive apart from thedrive shaft and the drive reel being omitted in addition to theretraction spring arrangement, whereby the interior of the clutch can berecognised, with the clutch being shown in the normal disengaged state,

FIG. 10 an axial view of a similar embodiment of the clutch and of thedrive means provided for it as likewise seen in the disengaged positionof the clutch,

FIG. 11 a similar perspective view to that of FIG. 9 but during theengagement procedure of the clutch,

FIG. 12 a partial longitudinal section of the embodiment of FIG. 10 butwith the belt winding reel being additionally shown,

FIG. 13 an end view of the subject of FIG. 12 with the additional driveremoved, but with the clutch of this embodiments also being illustratedduring the engagement procedure, and indeed shortly before the releaseof the resilient clutch disk,

FIG. 13a a section on the line XIIIa--XIIIa in FIG. 13.

FIG. 14 a similar perspective view to that of FIGS. 9 and 11, but withthe clutch being shown in the fully engaged state and the valvearrangement in the change-over position for actuation of the torquegenerating device,

FIG. 15 a partial longitudinal section analogous to that of FIG. 12 butwith the clutch disk being reproduced in the fully engaged state,

FIG. 16 an end view of the clutch of FIG. 15,

FIG. 17 a schematic sectional view of an additional drive in accordancewith the invention analogous to FIG. 5 but with only a flexible drawband,

FIG. 18 a schematic sectional view analogous to FIG. 17 of an embodimentof the present invention with two flexible draw bands,

FIG. 19 a perspective view of a particularly simple embodiment of atorque generating device for a belt tensioner in accordance with theinvention, with only one oppositely curved draw band and with the coverremoved, and

FIG. 20 a similar perspective view of that of FIG. 19 but with a sealingelement being mounted in the region of curvature of the"through-hanging" draw band.

In accordance with FIGS. 1 and 2 a belt tensioner has a holder 11 whichis for example to be mounted in the floor region of a passengercompartment to the vehicle chassis 40. A belt winding reel 12 isrotatably mounted on the holder 11 about a central axis 38 and a safetybelt 13 is wound onto the belt reel 12. The safety belt extends in themanner shown only in broken lines in the FIG. 1 via a belt lock 39 andfor example a wire cable 13' to another side of the vehicle chassis 40with the safety belt 13 normally being in contact with thenon-illustrated body of a vehicle occupant. Furthermore, FIG. 1indicates in chain-dotted lines at the top right that the safety belt 13can also be an element of a three point belt arrangement.

At the one end face of the belt winding reel 12 there is located acustomary retraction spring arrangement 14 in which a non-illustratedretraction spring is provided which exerts a light belt draw-in torqueon the belt winding reel 12 so that the safety belt which has beenpulled out and put on by the vehicle occupant rolls up onto the beltwinding reel 12 to such an extent that it loosely contacts the body ofthe vehicle occupant.

Furthermore, a customary but not illustrated belt pull-out blockingmechanism is in particular provided between the holder 11 and beltwinding reel 12 and becomes active with a sudden pull on the safety belt13 or during accelerations caused by an accident and fixes the beltwinding reel in the then prevailing pulled out or drawn in position sothat pulling out of the safety belt 13 is prevented.

The retraction spring arrangement can also be provided at the end faceof the belt winding reel 12 or at the holder 11 having the referencenumeral 14' in FIGS. 1 and 2. For this purpose an inner shaft 41 in FIG.1 must extend rotatably through a corresponding central bore of anadditional drive 15 and a clutch 20 which have yet to be described indetail in the following. The arrangement of the retraction springarrangement at 14' has the advantage that both the normal drive of thebelt winding reel 12 and also the additional drive of the belt runningreel 12 take place from the same end face.

In accordance with FIGS. 1 and 2 an additional drive 15 and a clutch 20are arranged between the retraction spring arrangement preferablyarranged at 14' and the holder 11 or the belt winding reel 12 and willbe described in the following in detail with reference to FIGS. 2 to 8.

As seen in FIGS. 1 and 3 the additional drive 15 has a drive shaft 19coaxial to the axis of rotation 38 of the belt winding reel 12 on whicha drive reel 22 is concentrically and rotationally fixedly secured. Inthe view of FIGS. 1 and 3 two draw bands 21 of spring steel sheet ofapproximately 0.2 mm thickness are wound onto the drive reel 22 aboveone another in the counter-clockwise sense and are rotationally fixedlysecured to the drive reel 22 at mounting points 25 on the periphery ofthe latter.

In the starting position shown in FIGS. 1 and 3 there are in each casethree to four layers of each draw band 21 wound onto the drive wheel 22.Thereafter each draw band 21 extends in arcuate manner through arespective one of the two band running chambers 23 which surround thedrive reel 22, to a respective mounting point 26 at the peripheral wall30 of the housing surrounding the band running chambers.

The arcuately shaped part of the draw bands 21 lies with its concavesurface in the starting position of the belt tensioner shown in FIG. 3against sealing walls 29 which are of curved shape in the radialdirection, are formed in one piece with the housing 24 and extend inscythe-like manner essentially radially inwardly to the layers of thedraw bands 21 wound onto the drive reel 22. There they form a taperedsealing lip 31 which ensures a sealed connection to the wound on drawbands 21.

In the region of the sealing lips 31 bar-like sealing elements 32 arearranged between each sealing wall 29 and the draw band 21 lying thereonand extend over the entire depth of the band running chambers 23 whichhave essentially planar end walls 51, 52. Whereas the planar end wall 51which has the one central throughbore 67 and is positioned adjacent tothe belt winding reel 12 forms a fixed component of the housing of thetorque generating device 18 in the same way as the peripheral wall 30,the axially oppositely disposed end wall 52 of FIG. 1 is located on alid 53 which can be mounted on the open side of the housing 24 and canalso be secured to it. The lid 53 has a central bore 54 for the passageof the inner shaft 41 to the retraction spring arrangement 14'.

The width of the draw bands 21 is so large that they sealingly andslidingly contacts the end walls 51, 52 and thus subdivide the bandrunning chambers 23 into two partial chambers 23', 23" sealed relativeto one another via the draw bands 21.

Branch lines 27 and 28 which respectively emerge from a common pressurechamber 17 energisable from a pressure source 16 open at the convex sideof the sealing walls 29 in the region of the roots of the sealing walls29 located at the peripheral wall 30. The pressure source 16 isconnected via a control line 42 indicated in broken lines in FIG. 3 toan acceleration sensor 43 which is likewise only indicated in brokenlines and which, in the case of a vehicle acceleration brought about byan accident, ignites a pyrotechnical charge in the pressure source 16 sothat a pressure can be abruptly built up in the pressure chamber 17.Vent openings 37 which lead to the surrounding atmosphere are providedat the concave side of the sealing walls 29 and prevent an excessivepressure build up in the partial chambers 23" on triggering of theadditional drive.

The branch lines 27, 28 open at the band running chambers 23 in such away that they can exert pressure on the draw bands 21 which extend atthe periphery of the band running chambers 23 transversely over theopenings of the branch lines 27, 28.

Through the described arrangement a torque generating device 18 isformed which, on triggering of the pyrotechnical charge 16, drives thedrive shaft 19 which is made hollow to receive the inner shaft 21, inthe belt winding direction as will be explained further below in detailin the description of the manner of operation.

A clutch 20 which is described in the following with reference to FIGS.1 and 9 to 16 is provided between the additional drive 15 with thetorque generating device 18 and the belt winding reel 12 and is normallydisengaged in order to ensure an unhindered interplay between theretraction spring arrangement 14' and the belt winding reel 12 via theinner shaft 41. Only when an accident dependent acceleration occurs andthe torque generating device 18 is intended to abruptly tension the belt13 must the clutch 20 engage in order to be able to transfer the torqueto the belt winding reel 12.

In accordance with FIGS. 1, 9 and 10 the drive reel 22 is provided atits side axially remote from the torque generating device 18 withaxially (FIG. 9) or radially (FIG. 10) projecting noses or wedge cams 55which engage in peripheral slots 56 of complementary shape at the outermargin of the central bore 71 of a clutch disk 44 in such a way that inthe peripheral direction a certain clearance admittedly exists betweenthe noses 55 and the boundaries of the peripheral slots 56 but such thatthe drive reel 22 causes the clutch disk 44 to execute a rotary movementwith it after overcoming this clearance.

At its outer periphery the clutch disk 44 has radially outwardlyprojecting holding noses or retaining cams 50 distributed around itsperiphery in accordance with FIGS. 1, 9 and 10 which engage inaccordance with FIGS. 10 and 12 behind counter-noses or cams 57 whichproject radially inwardly from the clutch housing 58 into the receivingcavity 60 for the clutch disk 44 in such a way that clutch disk 44 whichis pushed forwards in the direction of the belt winding reel 12 by theend face of the drive reel 22, or of an intermediate piece secured toit, is initially held radially outwardly against such an axialdisplacement. The clutch disk 44, which is made of resilient material,in particular of spring steel, is thereby conically resiliently deformedin accordance with FIGS. 12 and 13.

In accordance with the FIGS. 1, 9, 10 and 12 the clutch disk hasengagement cut-outs 45 which are uniformly distributed around theperiphery in its radially outer region but radially inwardly of theretaining noses 50. Engagement projections or cams 46 which projectaxially at the end guidance disk of the belt winding reel 12 whichaxially faces the torque generating device 18 are associated with theseengagement cut-outs 45 but are however out of engagement with theengagement cut-outs 45 (FIG. 12) in the position of the clutch disk 44in which its retaining noses 50 engage behind the counter noses 57.

As a result of the described arrangement the clutch disk 44 is normallyin the disengaged position which can be seen from FIG. 12 so that thebelt winding reel 12 can rotate freely unhindered by the drive reel 22and only cooperates with the retraction spring arrangement 14' via theinner shaft 41 which extends through the bores 67, 71 and 72.

In order to engage the clutch a displacement cut-out 49 is arranged inFIGS. 1, 9 and 10 alongside the clutch disk 44 in the clutch housing 58and extends substantially tangentially to the clutch disk 44. In thedisplacement cut-out 49 there is provided a slider 48 which likewiseextends tangentially and which is displaceable tangentially to theclutch disk 44 in the displacement cut-out 49.

The slider 48 has an actuating rod 48" at the top and an enlarged pistonregion 48' with a valve wall 59 which can be broken off at the bottom.Beneath the lower end face of the piston region 48' there is located thepyrotechnical charge 16. A pressure channel 61 branches off from thedisplacement cut-out 49 in the direction of the torque generating device18 (FIG. 12) and stands in flow connection with the pressure chamber 17.

With the slider 48 (FIGS. 9, 10 located in the starting position) thepiston region 48' blocks the connection between the pyrotechnical charge16 and the pressure channel 61.

In the upper region radially opposite to the clutch disk 44 theactuating rod 48" of the slider 48 has an abutment 62 which projects inthe direction towards the clutch disk 44 and engages with acounter-abutment 63 provided there in accordance with FIGS. 9 and 10 insuch a way that with a displacement of the slider 48 towards the top theclutch disk 44 is rotated in the clockwise sense.

The pyrotechnical charge 16 which is generally of cylindrical shape can,in accordance with FIGS. 1 to 4 and 9, be arranged at the side of theclutch 20 confronting the belt winding reel 12 parallel to and beneaththe belt winding reel 12 or however, in accordance with FIG. 10, in anaxial extension of the displacement cut-out 49 beneath the clutch 20. Itis only important that on triggering the pyrotechnical charge 16 it isfirst only the slider 48 and then, after the engagement of the clutch20, also the torque generating device 18 which is energised withpressure.

The manner of operation of the described belt tensioner is as follows:

When the acceleration sensor 43 (FIG. 3) detects an accident dependentacceleration at the vehicle it triggers the pyrotechnical charge 16 viathe control line 42. Thereafter the pyrotechnical charge does notinitially pressurise the torque generating device 18 as is shown byreason of simplified illustration in FIGS. 3 and 4 but rather, inaccordance with FIGS. 9 and 10, initially only the piston region 48' ofthe slider 48, so that the latter is displaced out of the position ofFIGS. 9, 10 into the position of FIGS. 11 to 13 and in so doing movesthe clutch disk 44 with it so that it executes a rotary movement.

In this way the holding noses or cams 50 of the clutch disk 44 aredisplaced in the peripheral direction along the stationary counter-nosesor cams 57 until they come out of engagement with one another in theposition of FIGS. 14 to 16. The path of displacement of the slider 48and the rotary movement of the clutch disk 44 are so calculated that inthe first stage of the movement the holding noses 50 and thecounter-noses 57 come out of engagement wherupon the clutch disk 44snaps in the direction of, i.e. towards the guidance disk 47. Duringthis the engagement projections 46 can engage into the engagementcut-outs 45 if these two elements should be coincidentally axiallyaligned. Should this not be the case then the webs 69 of the clutch disk44 contact the engagement noses 46. The clutch disk 44 is now turnedfurther in a second stage of the movement of the slider 48 until theengagement cut-outs 45 finally snap into place over the engagement noses46.

The peripheral slots 56 in the region of the central bore 72 must permita rotation of the clutch disk 44 relative to the drive reel 22 or to thenoses 55 which permits the two stages of movement.

As a result of the described design of the clutch 20 with numerousengagement cut-outs 45 distributed around the periphery and engagementnoses 46 account is taken of the need to the transmit considerabletorques via the clutch 20.

In detail the engagement of the clutch 20 proceeds as follows:

As can be seen from FIG. 1 the slider 48 which is advanced tangentiallyon ignition of the charge 16 rotates the clutch disk 44 for the purposeof engagement with the end guidance disk 47 precisely in the oppositedirection to that in which the clutch disk 44 is subsequently turned bythe torque generation device 18. These opposed directions of rotationare important, so that on rotation of the clutch disk 44 the drive shaft19 of the drive reel 22 is not turned at the same time by friction or byform-locked engagement. In the basic position prior to triggering of thepyrotechnical charge 16 the clutch disk 44 must stand relative to thedrive reel 22 in such a way that the entire free turning range of theclutch disk 44 relative to the drive reel 22 provided by the peripheralslots 56 is available.

A particularly important viewpoint of the invention lies in the way inwhich care is taken that the engagement cut-outs 45 and the engagementnoses 46 enter reliably into engagement during an engagement procedureof the clutch 20.

For this purpose the design is such that the abutment 62 on the slider48 and the counter-abutment 63 at the clutch disk 44 remain inengagement until the clutch disk 44 has snapped over the engagementnoses 46. In other words the abutments 62, 63 remain in engagement evenwhen the holding noses 50 have released from the counter-noses 57 on thecoupling housing 58 on rotation of the clutch disk 44 by the slider 48.In this case the resilient clutch disk 44 admittedly snaps out of theconical position which can be seen from FIG. 12 somewhat in thedirection of the belt winding reel 12 but only so far that--when theengagement noses 46 and the engagement cut-outs 45 are not yet axiallyaligned--the webs 69 between the engagement cut-outs 45 come intocontact on the engagement cams 46. The abutment 62 on the slider 42 isnow made sufficiently wide in the direction of the spring movement ofthe clutch disk 44 that it still remains in engagement with thecounter-abutment 63 even when the bayonet connection 50, 57 is releasedand the webs 69 have come into contact with the engagement noses 46. Inthis manner the clutch disk 44 is now turned on further by the continuedadvance of the slider 48 until the engagement cut-outs 45 snap over theengagement noses 46.

The clutch disk 44 now adopts the planar shape evident from FIG. 15which is shown in broken lines in FIG. 13a. In this position thecounter-abutment 63 is now axially separated from the abutment 62 of theactuating rod 48' so that the slider 48 and the clutch disk 44 are nowout of engagement and the slider 48 can now be advanced without theperformance of work only so far to reach the end of the displacementpath, until the valve wall 59 is located in front of the pressurechannel 61 and is then broken away by the pressure of the pressuresource 16 in order to free the flow path from the pressure source 16 tothe pressure chamber 17.

The invention thus not only leads to a mutual approach of the two clutchparts 44, 47, but also ensures a troublefree claw or sprague engagement.In FIG. 13a the direction of movement of the radially outer regions ofthe clutch disk 44 when the engagement cut-outs 45 snap over theengagement cams 46 is indicated by an arrow.

As a result of the design of the invention it is ensured that the clutch20 is reliably engaged before the torque generating device 18 issubjected to pressure from the pressure source 16.

The special advantage of the use of a resilient clutch disk, which isformed in the manner of a plate spring, lies in the fact that itrequires hardly any axial construction space and nevertheless enables areliable engagement and disengagement of the clutch 20.

As a result of its inherent spring characteristics the clutch disk 44which was initially conically deformed in accordance with FIG. 12 thussnaps finally into the planar position which can be seen from FIGS. 15and 16 in which the engagement noses 46 engage axially in the engagementcut-outs 45 and thus produce a rotationally fixed connection between thedrive reel 22 and the belt winding reel 12, which may, if necessary,have some circumferential clearance.

In this further advanced position of the slider 48 the frangible valvewall 59 has been pushed immediately before the entry to the pressurechannel 61 (FIGS. 11, 12).

It is now important that the lower margin 64 of the valve wall 59 islocated in the upper end position of the slider 48 somewhat above thelower boundary 65 of the pressure channel 61, whereby, as a result ofthe explosion pressure which is generated in the displacement cut-out 49in the direction of the pressure channel 61, a pressure is exerted onthe frangible valve wall 59 such that this breaks into the pressurechannel 61 as is illustrated in FIGS. 14 and 15.

The explosion pressure generated by the pyrotechnical charge 61 can nowpropagate via the pressure channel 61 into the pressure chamber 17 andfrom the latter out through the branch lines 27, 28 into the partialchambers 23'.

The pressure build up is illustrated in FIGS. 3 and 4 by dotted markingof the relevant spaces.

As a result of the pressure build-up in the branch lines 27, 28 acorresponding pressure force is generated on the concave side of thedraw bands 21 which brings about a torque at the drive reel 22 in thecounter-clockwise sense in FIG. 3.

In this way the drive reel 22 rotates while the unrolled tension band 21increasingly tightly contacts the peripheral wall 30 of the two bandrunning chambers 23.

As the free "through-hanging" or loop region of the draw band 21 isinitially comparatively short as a result of the special design of theband drawing running 23 of FIG. 4 the pressure force which istransmitted to the band is initially likewise relatively small.

If the band running chamber 23 then becomes larger, as shown in FIG. 4,then the effective "through-hanging" or loop surface of the draw band 21enlarges whereby the pressure force which is exerted on it risescorrespondingly and thus also the torque which is exerted on the drivereel 22.

In the further course of the movement the arcuate part of the draw band21 then enters into the end region of the band running chamber 23 whichbecomes smaller again whereby, at the end of the belt tensioning a lowerdrive force is ultimately exerted on the drive reel 22.

At the conclusion of the movement the curved part of the draw band 21lies against a concavely curved end region 66 of the band runningchamber 23. The movement clearance of the draw bands 21 is now exhaustedand the belt 13 is drawn in by the maximum amount of for example 205 mm.

In the position of the draw bands 21 of FIG. 4 the drawn-in length ofthe safety belt 13 amounts, up to this point, to about 140 mm.

If now the pressure force in the partial chambers 23' drops off due tothe pyrotechnical charge being used up then the one to two windings ofthe spring steel draw band present on the drive reel 22 become loose,whereupon the spring tongue 35 which is pressed in accordance with FIG.7 into a peripheral recess 36 by the tension exerted on the draw band 21presses the loose layers of the draw band 21 outwardly as a result ofits inherent spring force and adopts the relieved position evident fromFIG. 8 in which the engagement between the end of the draw band 21 whichis equipped with the spring tongue 35 and the drive reel 22 whichresults in the co-movement is cancelled.

The drive wheel 22 can now rotate unhindered under the influence of drawout and draw in movements of the safety belt 13 while the clutch 20remains closed. In this way the normal function of the belt unwindingand winding up mechanism by means of the retraction spring arrangement14' is ensured now as before.

If the belt 13 is for example ideally tensioned in the position of FIG.4 or with the draw bands 21 unrolled still further, and if the pressureof the pyrotechnical charge 16 lets up, then the non-illustratedblocking device between the holder 11 and the belt winding reel 12blocks a draw out movement now as previously which is important becausethe vehicle occupant falls into the belt 13 after the abrupt tensioningof the same and a drawing out of the belt must be reliably prevented.

The invention thus provides an extremely compact belt tensioner in whichfirst the engagement of the clutch and then the tensioning of the beltis ensured by one and the same pyrotechnical charge.

The embodiment of FIGS. 5 and 6 is distinguished from that of FIGS. 1 to4 in that the two draw bands 21 which are only shown in broken lines areunited into a single continuous band and guided through a band channel33 outside of the mounting points 26 at the housing.

For this reason the forces which occur at the draw bands 21 in theregion of the mounting points 26 on pressure loading of the torquegenerating device 18 are only transmitted to the housing 24 to theextent that the channels which accommodate the band 21 are curved. Theactual reaction forces cancel within the band region 21' located in thepressure channel 23.

In accordance with FIG. 5 the draw band 21 passes at the top left in theregion of the mounting point 26 transversely through a cylindricalpressure space 17, whereby the region 21" of the draw band 21 whichpasses through this pressure space 17 subdivides the space into twopartial spaces 17', 17".

The region 21' of the draw band 21 which extends in the pressure channel33 serves solely to mutually support the parts of the draw bands 21located in the band running chambers 23.

In accordance with FIG. 5 a groove 34 can also be provided at the convexside of the sealing walls 29 which ensures that at the start ofoperation, when the draw bands 21 still lie tightly on the convex sideof the sealing walls 29, the pressure coming from the pressure chamber17 immediately propagates over the full radial extent of the draw bands21.

In FIG. 5 it is also shown that a radially inwardly disposed region ofthe sealing walls 29 can be pivotally connected to the other part in thedirection of the arrow about a hinge 74.

In this way, following triggering, the inner sealing wall region whichcan be pivoted away is pivoted away whereby the automatic disengagementof the band ends from the peripheral recesses in the drive reel 22described with reference to FIGS. 7 and 8 is also not hindered in theregion of the sealing walls 29.

In order that the radially inner part of the sealing walls 29 of FIG. 5pivots away in the direction of the arrows with a pressure reduction,following tensioning, a non-illustrated spreading spring must beprovided between the two mutually pivotable parts of the sealing wall29.

A spreading spring of this kind can be avoided in the embodiment of FIG.5a if the sealing walls 29 are held by the tension of the draw band 21lying on them via a suitably shaped partition location 75 and via anabutment 24' fixed relative to the housing. As a result of this designthe sealing walls 29 are pressed in the normal state against thepartition joint 75 and against the abutment 24' by a suitableprestressing of the drive reel 22 by the draw band 21 standing undertension.

On triggering of the pyrotechnical charge the pressure present in thepartial space 23' then presses the sealing walls 29 against thepartition joint 75 and the abutment 24'. Only when the draw bands 21have largely unwound from the drive wheel 22, and the pressure in thepartial chambers 23' has dropped off again, do the now relaxing layersof the draw bands 21 on the drive wheel 22 press against the sealingwalls 29 so that they slide along the partition joint 75 and theabutment 24' in the direction of the arrow 73 and thus not prevent therelaxation of the layers of the draw bands 21 on the drive wheel 22. Inthis manner the ends of the draw bands 21 which are rotationally fixedlyhung in to the periphery of the drive reel 22 automatically resilientlyrelease from the recesses into which they are hung when the hanging inpoints are located directly opposite to the sealing lip 31 of a sealingwall 29.

The inventive concept of this embodiment is thus to be seen in the factthat the sealing walls 29 are so releasably mounted on the housing 24that they are kept in their desired position by the draw band 21 whichstands under tension or by the pressure in the palatial space 23' but,when no longer contacted by the draw band 21 and with falling pressureautomatically release from the housing 24, 24' through the relaxingcoils of the draw bands 21 on the drive reel 22.

The mounting bores 70 illustrated in FIG. 5 serve for the axial clampingof the individual housings of the additional drive 15, clutch 20 andretraction spring arrangement 14' and also of the frame 11 of the beltwinding reel 12.

Through the design of the invention the tensioning of the safety belt by140 to 205 mm can be achieved within approximately 10 msec after thetriggering of the pyrotechnical charge.

The draw bands 21 are wound three to four times around the drive shaft19 or of the drive reel 22 in the starting position. The diameter of thedrive shaft 19 or the drive reel 22 and the number of the turns of thedraw bands thereon depend on the desired draw-in length duringtensioning. Moreover, attention must be paid to the fact that after theband loops have run fully through the band running space 23 one to twoturns are present on the drive shaft 19 or the drive reel 22 so that theautomatic movement of the band ends out of the peripheral recess 36(FIG. 7) does not occur prematurely. The at least one to two turns onthe drive shaft 19 or the drive reel 22 are thus necessary in order toensure that the spring tongue 35 or other hanging-in or fixing member isreliably pressed into a corresponding recess 36 of the drive shaft 19 ofthe drive wheel 22 so long as tension is present on the draw bands 21.

Whereas, instead of two diametrically oppositely disposed draw bands 21which operate on one and the same drive shaft 19 or drive reel 22,several such arrangements which are distributed around the peripherycould fundamentally be used, the use of more than four draw bands 21distributed around the periphery with corresponding sealing walls 29cannot be considered for constructional reasons. Ideal is in any eventthe use of two diametrically oppositely disposed draw bands 21 withcorresponding sealing walls 29, because in this way the bearing forceson the drive shaft 9 or on the drive reel 22 are already hereby largelyovercome.

In the embodiment of FIG. 17 the flexible draw band 21 extends with thesame sense of curvature which it has at the drive reel 22 through theband running chamber 23 to the peripheral wall 30 which lies radiallyopposite to the drive reel 22 and preferably concentric to the driveshaft 19 where it is secured to the housing 24 in the region of theopening of an explosion pressure feedline 27 in the band running chamber23 at 26.

Between the housing 24 and the periphery of the drive reel 22, or of thedraw band 21 wound thereon, a sealing wall 29 extends substantiallyparallel to the feed line 27 and close to the latter to the periphery ofthe drive reel 22 where a sealing lip 29' is provided which sealinglycontacts the coiled up draw band 21. In this manner the partial chambers23', 23" are decoupled from one another pressurewise At the peripheralend of the band running chamber 23 in the propagation direction of theflexible draw band 21 there is located apart from the sealing wall 29also the vent bore 37 which leads a vent channel 37' into thesurrounding atmosphere.

As shown in chain-dotted lines and in fine hatching in FIG. 17 thesealing wall 29 preferably has a projection 29" towards the partialchamber 23' so that the sealing lip 29' is somewhat lengthened in thedirection of the pressure chamber 23'. The projection 29" is of flexibledesign and could also be hingedly secured to the actual sealing wall 29,and indeed about a pivot axis parallel to the axis of the drive shaft19. In this manner the sealing lip 29' is pressed by the pressure in thepartial chamber 23 particularly well in sealing manner against the outersurface of draw band 21 wound onto the drive reel 22.

FIG. 18 shows a similar embodiment to that of FIGS. 3 and 4 however theflexible draw bands 21 extend, similarly to the embodiment of FIG. 17,without reversal of the direction of curvature which they have on thedrive reel 22, through the band running chambers 23 to the peripheralwall 30 in order to be secured there to the housing 24 at 26.

In this embodiment the partial chambers 23" are also vented towards thesurrounding atmosphere at the peripheral end through a vent bore 37 anda subsequent vent channel 37'.

The important advantage of the embodiment of FIG. 17 and 18 lies in thefact that the pressure in the pressure charged partial chamber 23' canpropagate up to the sealing lip 29' of the sealing wall of sealing walls29 so that the region of the flexible draw band 21 which extends freelythrough the band running chambers 23, and is responsible for the tensionforce generation brought about by the pressure loading, can beconsiderably enlarged relative to the embodiment of FIGS. 3 and 4, andindeed, in the present embodiment, by the diameter of the drive reel 22.With the same pressure the tension force of the flexible draw band 21which sets the drive reel 22 in motion is thus decisively increased.

In FIG. 18 the flexible draw band is shown in continuous lines at thestart of pressure loading and also in broken lines in two furtheradvanced stages of a pressure release in which the safety belt isalready more strongly tensioned.

Particularly important is the longitudinal extent of the sealing walls29 essentially tangential to the drive reel 22. Furthermore, the sealingwalls 29 are flexible in accordance with the invention, i.e. are inparticular of elastically resilient design or are for example hingedlysecured to the housing 24 at 29'", with the arrangement being such thatthrough small spring forces the sealing walls 29 can be so biased in thedirection towards the flexible draw band 21 wound onto the drive reel 22that the sealing lips 29' are lightly pressed against the outer surfaceof the wound up draw band 21.

In this manner each pressure loading of the surface of the sealing walls29 confronting the partial chambers 23 causes the sealing lip 29' of thesealing walls to be pressed against the outer surface of the draw band21 wound onto the drive wheel 22, whereby a sealing action whichincreases with the pressure in the partial chamber 23' is achievedbetween the sealing lips 29' and the drive reel 22 or the draw band 21wound onto it.

Furthermore, in advantageous manner, the draw band 21 is also pressedradially from the inside towards the outside against the sealing lip 29'by the pressure which acts on it in the associated partial space 23'.

A further embodiment is shown in FIG. 19 which only reproduces theimportant part of the belt tensioner. The belt tensioner has a draw band21 which extends over the width of the band running chamber 23 as theforce transmitting sealing means in a housing 24. The draw band 21 issecured at its one end to a mounting point 25 at the drive reel 22. Thedraw band is wound partially onto the drive reel in a direction suchthat when drawing off the draw band 21 the drive reel 22 rotates in thebelt winding up direction and is guided from the radially outer end ofthe wound up part from the drive reel 22 into a 180° arc. This arc whichpoints away from the pressure feedline 27 towards the smoothly formedperipheral wall 30 of the housing 24 which it contacts and is then ledto a mounting point 26 at the start of the band running chamber 23adjacent to the feedline 27 where its other end is secured.

The housing 24 is formed similarly to that of FIG. 1 with a planar endwall 51 and a planar cover 53 which are contacted inwardly by the edgesof the draw band 21 so that in the direction of rotation the pressureloaded partial chamber 23' is formed before the 180° arc and the partialchamber 23" vented at 37 is formed behind the 180° arc.

The clutch 20 which is only indicated in broken lines and which isnormally disengaged and engage in non-illustrated manner with accidentdependent accelerations must be imagined to be present between the drawband 21 and the drive shaft 19 which must be imagined as beingrotationally fixedly connected to the belt reel and which projectsaxially from the drive wheel 22. The clutch is however preferablyprovided between the drive shaft 19 and the belt reel 15.

The embodiment shown in FIG. 20 has an additional sealing cylinder 76 inthe 180° arc of the draw band 21 with the flat end faces of the sealingcylinder 76 standing in sliding sealing engagement with the wall 51 andthe cover 53. In this way a better end face sealing of the two parts ofthe band running chamber 23 is achieved.

In both cases the clutch 20 should be designed so that the engagement iscancelled again after an adequate time following the accident dependenttensioning. A preferred example for a clutch was described above withreference to FIGS. 1 and 13 to 16.

The manner of operation of the embodiments shown in FIGS. 19 and 20 isas follows:

In the starting position the draw band 21 is wound up by a maximumamount onto the drive wheel 22 and the 180° arc is located close to andopposite to the feedline 27. In this manner the pressure chamber 23'which is present between the draw band 21, the pressure sealed sealingwall 29, the peripheral wall 30 and also the end walls 51, 53, and whichcan be charged from the feedline 27, has its minimum volume.

In the starting position it is however also possible that the draw band21 does not have a 180° arc but rather passes in an essentially straightline from the drive reel 22 to the peripheral wall 30 to the mountingpoint 26 or close to it.

If then an acceleration caused by an accident occurs the pressure sourceis triggered and the clutch 20 engages between the draw band 21 and thenon-illustrated belt winding mechanism.

The driving gases from the pyrotechnical charge flow through the drawband 21 into the pressure space 23' so that the pressure here is largerthan in the part of the running chamber remote from the pressure source.This pressure difference brings about a force which is directed in theperipheral direction away from the draw band 27 towards the 180° arc ofthe draw band 21 whereby, since the draw band is held at the housing atthe mounting point 26, a counter-clockwise torque is exerted onto thedrive shaft 22 and the drive shaft 22 is caused to execute acorresponding rotary co-movement. With this direction of rotation of thedrive shaft 22 the non-shown belt reel is turned via the clutch 20 intothe belt tensioning direction and the draw band 21 which is wound ontothe drive reel and held fast at its other end at the mounting point 26is simultaneously unwound. Under the pressure difference the draw band21 in the partly circular band running chamber 23 follows a path whichbecomes larger during this procedure starting from the mounting point26. The 180° arc thereby runs through the band running chamber 23 in thecounter-clockwise sense.

The FIGS. 19 and 20 show the belt tensioner shortly after the start ofthe pressure loading where the 180° arc has first moved through asmaller part of the band running chamber 23. In the event that the drawband 21 is guided in the starting position in a straight line from thedrive shaft 22 to the peripheral wall 30 close to the mounting point 26,the 180° arc will first form at the start of the pressure loading withan unwinding rotation of the hub 22 and will then move through the bandrunning chamber 23 as described above.

The sealing cylinder 76 of FIG. 20 executes a rolling off movement onthe band part which follows the peripheral wall 30 during the movementof the 180° arc through the band running chamber 23. In this case thepressure force is largely transmitted via the sealing cylinder 76 to the180° arc.

The distance by which the draw band 21 is unwound correspondsessentially to the product of the running angle of the apex of the 180°arc (in radians) and the peripheral wall diameter. The angle of rotationthrough which the drive shaft 22 is turned results from the quotient ofthis parameter and the drive roll diameter. Accordingly the angle ofrotation of the drive roll is enlarged by the ratio of the peripheraldiameter to the drive roll diameter.

We claim:
 1. Belt tensioner for safety belt arrangements in motorvehicles having a holder (11) secured to the vehicle chassis and on theholder a belt winding reel (12) with a safety belt (13) wound thereon, aretraction spring arrangement (14, 14') by means of with the safety belt(13) is continuously and lightly so pretensioned in the retractiondirection that it only lies loosely on the vehicle occupant secured byit, a blocking arrangement between the belt winding reel (12) and theholder (11) of the vehicle chassis which blocks the safety belt (13)against being pulled out if suddenly pulled, and/or in the event of anacceleration resulting from an accident, and an additional drive (15)connected to the belt winding reel (12) via a clutch (20), wherein theadditional drive (15) is normally separated drivewise from the beltwinding reel (12) by the disengaged clutch (20), but is abruptlyactivated in the event of a acceleration resulting from an accident andis rotationally fixedly coupled with the belt winding reel (12) via thethen engaged clutch (20) in order, before the vehicle occupantaccelerates against the safety belt (13), to wind up such a length ofthe safety belt that the safety belt lies tightly on the vehicleoccupant, with the additional drive (15) having a pressure source whichis triggered or ignited by the accident dependent response of anacceleration sensor and which, on being triggered, abruptly builds up ahigh pressure in a pressure chamber (17), a torque generating device(18) which is energised by the pressure in the pressure chamber (17) anda drive shaft (19) connected to the torque generating device (18) whichis rotationally fixedly connected with the belt winding reel (12) viathe clutch (20), characterised in that the torque generating device (18)has at least one flexible tension band (21) which is secured at one endat a first mounting point (25) to the periphery of the drive shaft (19),or to the drive reel (22) which is concentrically mounted thereon, andis partially wound onto the drive shaft (19), or onto the drive reel(22), and is secured at the other end at second mounting point (26) tothe peripheral wall (30) of a band running chamber (23) which isarranged within a housing (24) around the drive shaft (19), or aroundthe drive reel (22) and has a width corresponding to the width of thedraw band (21) in such a way that the draw band (21) extends through theband running chamber (23) while subdividing the band running chamber(23) into two partial chambers (23', 23") sealed off relative to oneanother; and in that the partial chamber (23') which on being subjectedto pressure brings about an unwinding of the tension band (21) from therotating drive shaft (19) or drive reel (22) is connected to thepressure space (17) and with the other partial space (23") is vented;and in that for the pressure-wise decoupling of the partial chambers(23', 23"), at least one sealing wall (29) is provided which extendsbetween the housing (24) and the periphery of the flexible draw band(21) which is wound onto the drive shaft (19), or onto the drive reel(22) concentrically mounted thereon wherein the flexible draw band (21)extends through the band running chamber (23) up to its peripheral wall(30) with the same direction of curvature which it has as a result ofbeing wound onto the drive shaft (19) or onto the drive reel (22). 2.Belt tensioner for safety belt arrangements in motor vehicles having aholder (11) secured to the vehicle chassis and on the holder a beltwinding reel (12) with a safety belt (13) wound thereon, a retractionspring arrangement (14, 14') by means of with the safety belt (13) iscontinuously and lightly so pretensioned in the retraction directionthat it only lies loosely on the vehicle occupant secured by it, ablocking arrangement between the belt winding reel (12) and the holder(11) of the vehicle chassis which blocks the safety belt (13) againstbeing pulled out if suddenly pulled, and/or in the event of anacceleration resulting from an accident, and an additional drive (15)connected to the belt winding reel (12) via a clutch (20), wherein theadditional drive (15) is normally separated drivewise from the beltwinding reel (12) by the disengaged clutch (20), but is abruptlyactivated in the event of a acceleration resulting from an accident andis rotationally fixedly coupled with the belt winding reel (12) via thethen engaged clutch (20) in order, before the vehicle occupantaccelerates against the safety belt (13), to wind up such a length ofthe safety belt that the safety belt lies tightly on the vehicleoccupant, with the additional drive (15) having a pressure source whichis triggered or ignited by the accident dependent response of anacceleration sensor and which, on being triggered, abruptly builds up ahigh pressure in a pressure chamber (17), a torque generating device(18) which is energised by the pressure in the pressure chamber (17) anda drive shaft (19) connected to the torque generating device (18) whichis rotationally fixedly connected with the belt winding reel (12) viathe clutch (20), characterised in that the torque generating device (18)has at least one flexible tension band (21) which is secured at one endat a first mounting point (25) to the periphery of the drive shaft (19),or to the drive reel (22) which is concentrically mounted thereon, andis partially wound onto the drive shaft (19), or onto the drive reel(22), and is secured at the other end at a second mounting point (26) tothe peripheral wall (30) of a band running chamber (23) which isarranged within a housing (24) around the drive shaft (19), or aroundthe drive reel (22) and has a width corresponding to the width of thedraw band (21) in such a way that the draw band (21) extends through theband running chamber (23) while subdividing the band running chamber(23) into two partial chambers 23") sealed off relative to one another;and in that the partial chamber (23') which on being subjected topressure brings about an unwinding of the tension band (21) from therotating drive shaft (19) or drive reel (22) is connected to thepressure space (17) and with the Other partial space (23") is vented;and in that for the pressure-wise decoupling of the partial chambers(23', 23"), at least one sealing wall (29) is provided which extendsbetween the housing (24) and the periphery of the flexible draw band(21) which is wound onto the drive shaft (19), or onto the drive reel(22) concentrically mounted thereon wherein the sealing wall or walls(29) has or have a sealing lip (29') facing the outer surface of thecoiled up draw band (21) and so extends or extend relative to thehousing, and is or are so flexibly or hingedly designed or arranged,that the pressure in the associated pressure chamber (23') sealinglypresses the sealing lip (29') against the outer surface of the coiled updraw band (21).
 3. Belt tensioner for safety belt arrangements in motorvehicles having a holder (11) secured to the vehicle chassis and on theholder a belt winding reel (12) with a safety belt (13) wound thereon, aretraction spring arrangement (14, 14') by means of with the safety belt(13) is continuously and lightly so pretensioned in the retractiondirection that it only lies loosely on the vehicle occupant secured byit, a blocking arrangement between the belt winding reel (12) and theholder (11) of the vehicle chassis which blocks the safety belt (13)against being pulled out if suddenly pulled, and/or in the event of anacceleration resulting from an accident, and an additional drive (15)connected to the belt winding reel (12) via a clutch (20), wherein theadditional drive (15) is normally separated drivewise from the beltwinding reel (12) by the disengaged clutch (20), but is abruptlyactivated in the event of a acceleration resulting from an accident andis rotationally fixedly coupled with the belt winding reel (12) via thethen engaged clutch (20) in order, before the vehicle occupantaccelerates against the safety belt (13), to wind up such a length ofthe safety belt that the safety belt lies tightly on the vehicleoccupant, with the additional drive (15) having a pressure source whichis triggered or ignited by the accident dependent response of anacceleration sensor and which, on being triggered, abruptly builds up ahigh pressure in a pressure chamber (17), a torque generating device(18) which is energised by the pressure in the pressure chamber (17) anda drive shaft (19) connected to the torque generating device (18) whichis rotationally fixedly connected with the belt winding reel (12) viathe clutch (20), characterised in that the torque generating device (18)has at least one flexible tension band (21) which is secured at one endat a first mounting point (25) to the periphery of the drive shaft (19),or to the drive reel (22) which is concentrically mounted thereon, andis partially wound onto the drive shaft (19), or onto the drive reel(22), and is secured at the other end at a second mounting point (26) tothe peripheral wall (30) of a band running chamber (23) which isarranged within a housing (24) around the drive shaft (19), or aroundthe drive reel (22) and has a width corresponding to the width of thedraw band (21) in such a way that the draw band (21) extends through theband running chamber (23) while subdividing the band running chamber(23) into two partial chambers (23', 23") sealed off relative to oneanother; and in that the partial chamber (23') which on being subjectedto pressure brings about an unwinding of the tension band (21) from therotating drive shaft (19) or drive reel (22) is connected to thepressure space (17) and with the other partial space (23") is vented;and in that for the pressure-wise decoupling of the partial chambers(23', 23"), at least one sealing wall (29) is provided which extendsbetween the housing (24) and the periphery of the flexible draw band(21) which is wound onto the drive shaft (19), or onto the drive reel(22) concentrically mounted thereon wherein the radial extent of theband running chamber varies in dependence on the angle of rotationrelative to the axis of rotation of the drive shaft (19) or of the drivereel (20).
 4. Belt tensioner in accordance with claim 3, characterisedin that the radial extent of the band running chamber (23") firstincreases in the direction of rotation which arises on the applicationof pressure--starting from the starting position of the draw band (21)corresponding to the maximum wound up position on the drive shaft (19),or the drive reel (20)--and then remains at least approximately constantor reduces again.
 5. Belt tensioner in accordance with claim 3,characterised in that the sealing walls (29) taper in scythe-like mannertowards the drive shaft (19), or towards the drive reel (22), and form asealing lip (31) radially inwardly with the draw bands (21) wound ontothe drive shaft (19), or the drive reel (22).
 6. Belt tensioner forsafety belt arrangements in motor vehicles having a holder (11) securedto the vehicle chassis and on the holder a belt winding reel (12) with asafety belt (13) wound thereon, a retraction spring arrangement (14,14') by meads of with the safety belt (13) is continuously and lightlyso pretensioned in the retraction direction that it only lies loosely onthe vehicle occupant secured by it, a blocking arrangement between thebelt winding reel (12) and the holder (11) of the vehicle chassis whichblocks the safety belt (13) against being pulled out if suddenly pulled,and/or in the event of an acceleration resulting from an accident, andan additional drive (15) connected to the belt winding reel (12) via aclutch (20), wherein the additional drive (15) is normally separateddrivewise from the belt winding reel (12) by the disengaged clutch (20),but is abruptly activated in the event of a acceleration resulting froman accident and is rotationally fixedly coupled with the belt windingreel (12) via the then engaged clutch (20) in order, before the vehicleoccupant accelerates against the safety belt (13), to wind up such alength of the safety belt that the safety belt lies tightly on thevehicle occupant, with the additional drive (15) having a pressuresource which is triggered or ignited by the accident dependent responseof an acceleration sensor and which, on being triggered, abruptly buildsup a high pressure in a pressure chamber (17), a torque generatingdevice (18) which is energised by the pressure in the pressure chamber(17) and a drive shaft (19) connected to the torque generating device(18) which is rotationally fixedly connected with the belt winding reel(12) via the clutch (20), characterised in that the torque generatingdevice (18) has at least one flexible tension band (21) which is securedat one end at a first mounting point (25) to the periphery of the driveshaft (19), or to the drive reel (22) which is concentrically mountedthereon, and is partially wound onto the drive shaft (19), or onto thedrive reel (22), and is secured at the other end at a second mountingpoint (26) to the peripheral wall (30) of a band running chamber (23)which is arranged within a housing (24) around the drive shaft (19), oraround the drive reel (22) and has a width corresponding to the width ofthe draw band (21) in such a way that the draw band (21) extends throughthe band running chamber (23) while subdividing the band running chamber(23) into two partial chambers (23', 23") sealed off relative to oneanother; and in that the partial chamber (23') which on being subjectedto pressure brings about an unwinding of the tension band (21) from therotating drive shaft (19) or drive reel (22) is connected to thepressure space (17) and with the other partial space (23") is vented;and in that for the pressure-wise decoupling of the partial chambers(23', 23"), at least one sealing wall (29) is provided which extendsbetween the housing (24) and the periphery of the flexible draw band(21) which is wound onto the drive shaft (19), or onto the drive reel(22) concentrically mounted thereon wherein a plurality of draw bands(21) and associated band running chambers (23) are provided distributedover the periphery of the drive shaft (19), or of the drive reel (22),and are separated from one another in pressure-tight manner by sealingwalls (29).
 7. Belt tensioner in accordance with claim 6, characterisedin that two draw bands (21) and associated band running chambers (23)are provided at equal angular distances.
 8. Belt tensioner in accordancewith claim 6, characterised in that the several band running chambers(23) are connected via suitable branch lines (27, 28) to a singlepressure chamber (17) or to a single pressure source (16).
 9. Belttensioner in accordance with claim 6, characterised in that the sealingwalls (29) extend from the peripheral wall (30) to the periphery of thedrive shaft (19), of the drive reel (22), and are curved in such a waythat the part of the draw band (21) extending between the drive shaft(19), or the drive reel (20), and the peripheral wall (30) contacts thecurved sealing wall (29) in the starting position and adopts a concavelycurved shape relative to the partial chamber (23') which is subjected topressure.
 10. Belt tensioner in accordance with claim 9, characterisedin that the part of the draw band (21) which extends between the driveshaft (19), or the drive reel (22), and the peripheral wall (30)contacts, in the end position, against the curved sealing wall (29)which follows in the direction of rotation.
 11. Belt tensioner inaccordance with claim 6, characterised in that, sealing elements (32)are arranged at the pressure side of the draw bands (21), in the regionof the smallest spacing between the draw bands (21) wound onto the driveshaft (19), or onto the drive reel (22), and the sealing wall (29), eachsaid sealing element extending transversely over the band running space(23), with the thickness of each sealing element being sufficientlylarge that it sealingly contacts the gap between the periphery of therespective coiled up draw band and the sealing wall through the pressurein the partial chamber (23") to which pressure is applied, even with themaximum gap present in the largely uncoiled state of the draw band (21),but is not however pressed through the gap.
 12. Belt tensioner inaccordance with claim 6, characterised in that the draw bands (21) whichfollow one another in the direction of rotation do not end in the regionof their mounting points (26) at the housing (24) but join with eachother outside of the band running chamber (23) to cooperate to form asingle long continuous band.
 13. Belt tensioner in accordance with claim12, characterised in that a band channel (33) extends in the housing(24) between the neighbouring mounting points (26), with the part (21')of the long band (21, 21', 21") which connects the actual draw bands(21) within the housing (24) extending through the band channel (23).14. Belt tensioner in accordance with claim 6, characterised in that thesealing wall (29) has at least one gas conducting groove (34) extendingfrom radially outwardly to radially inwardly at the surface confrontingthe partial space (23') subjected to pressure.
 15. Belt tensioner forsafety belt arrangements in motor vehicles having a holder (11) securedto the vehicle chassis and on the holder a belt winding reel (12) with asafety belt (13) wound thereon, a retraction spring arrangement (14,14') by means of with the safety belt (13) is continuously and lightlyso pretensioned in the retraction direction that it only lies loosely onthe vehicle occupant secured by it, a blocking arrangement between thebelt winding reel (12) and the holder (11) of the vehicle chassis whichblocks the safety belt (13) against being pulled out if suddenly pulled,and/or in the event of an acceleration resulting from an accident, andan additional drive (15) connected to the belt winding reel (12) via aclutch (20), wherein the additional drive (15) is normally separateddrivewise from the belt winding reel (12) by the disengaged clutch (20),but is abruptly activated in the event of a acceleration resulting froman accident and is rotationally fixedly coupled with the belt windingreel (12) via the then engaged clutch (20) in order, before the vehicleoccupant accelerates against the safety belt (13), to wind up such alength of the safety belt that the safety belt lies tightly on thevehicle occupant, with the additional drive (15) having a pressuresource which is triggered or ignited by the accident dependent responseof an acceleration sensor and which, on being triggered abruptly buildsup a high pressure in a pressure chamber (17), a torque generatingdevice (18) which is energised by the pressure in the pressure chamber(17) and a drive shaft (19) connected to the torque generating device(18) which is rotationally fixedly connected with the belt winding reel(12) via the clutch (20), characterised in that the torque generatingdevice (18) has at least one flexible tension band (21) which is securedat one end at a first mounting point (25) to the periphery of the driveshaft (19), or to the drive reel (22) which is concentrically mountedthereon, and is partially wound onto the drive shaft (19), or onto thedrive reel (22), and is secured at the other end at a second mountingpoint (26) to the peripheral wall (30) of a band running chamber (23)which is arranged within a housing (24) around the drive shaft (19), oraround the drive reel (22) and has a width corresponding to the width ofthe draw band (21) in such a way that the draw band (21) extends throughthe band running chamber (23) while subdividing the band running chamber(23) into two partial chambers (23', 23") sealed off relative to oneanother; and in that the partial chamber (23') which on being subjectedto pressure brings about an unwinding of the tension band (21) from therotating drive shaft (19) or drive reel (22) is connected to thepressure space (17) and with the other partial space (23") is vented;and in that for the pressure-wise decoupling of the partial chambers(23', 23"), at least one sealing wall (29) is provided which extendsbetween the housing (24) and the periphery of the flexible draw band(21) which is wound onto the drive shaft (19), or onto the drive reel(22) concentrically mounted thereon wherein the end of the draw band ordraw bands (21) located at the drive shaft (19) or drive reel (22) isnormally kept engaged in the drive shaft (19), or in the drive reel (22)by the layers of the draw bands (21) which stand under tension on thedrive shaft (19) or drive reel (22) and can be automatically disengagedin the radial direction on a drop in pressure following triggering, withthe tension prior to triggering in the normal state being ensured by aretaining means which holds the drive shaft (19) or the drive reel (22)in the tensioned state until triggered and is made inactive by thepressure generated in the pressure chamber (17) on triggering of thepressure source.
 16. Belt tensioner in accordance with claim 15,characterised in that a spring tongue (35) provided at the draw band endis pressed by the layers of the draw band (21) wound above it into aperipheral recess (36) of the drive shaft (19), or drive reel (22), andin this way a form-locked drive connection is produced between the drawband (21) and the drive shaft (19), or drive wheel (22), and in that thespring tongue also emerges from the peripheral recess (36) with theunwinding of the draw band (21) corresponding to belt tensioning and thetermination of the pressure loading, as a result of the inherentresilient resetting force it possesses, and thus cancels the form lockeddrive connection between the draw band (21) and the drive shaft (19) ordrive reel (22).
 17. Belt tensioner for safety belt arrangements inmotor vehicles having a holder (11) secured to the vehicle chassis andon the holder a belt winding reel (12) with a safety belt (13) woundthereon, a retraction spring arrangement (14, 14') by means of with thesafety belt (13) is continuously and lightly so pretensioned in theretraction direction that it only lies loosely on the vehicle occupantsecured by it, a blocking arrangement between the belt winding reel (12)and the holder (11) of the vehicle chassis which blocks the safety belt(13) against being pulled out if suddenly pulled, and/or in the event ofan acceleration resulting from an accident, and an additional drive (15)connected to the belt winding reel (12) via a clutch (20), wherein theadditional drive (15) is normally separated drivewise from the beltwinding reel (12) by the disengaged clutch (20), but is abruptlyactivated in the event of a acceleration resulting from an accident andis rotationally fixedly coupled with the belt winding reel (12) via thethen engaged clutch (20) in order, before the vehicle occupantaccelerates against the safety belt (13), to wind up such a length ofthe safety belt that the safety belt lies tightly on the vehicleoccupant, with the additional drive (15) having a pressure source whichis triggered or ignited by the accident dependent response of anacceleration sensor and which, on being triggered, abruptly builds up ahigh pressure in a pressure chamber (17), a torque generating device(18) which is energised by the pressure in the pressure chamber (17) anda drive shaft (19) connected to the torque generating device (18) whichis rotationally fixedly connected with the belt winding reel (12) viathe clutch (20), characterised in that the torque generating device (18)has at least one flexible tension band (21) which is secured at one endat a first mounting point (25) to the periphery of the drive shaft (19),or to the drive reel (22) which is concentrically mounted thereon, andis partially wound onto the drive shaft (19), or onto the drive reel(22), and is secured at the other end at a second mounting point (26) tothe peripheral wall (30) of a band running chamber (23) which isarranged within a housing (24) around the drive shaft (19), or aroundthe drive reel (22) and has a width corresponding to the width of thedraw band (21) in such a way that the draw band (21) extends through theband running chamber (23) while subdividing the band running chamber(23) into two partial chambers (23', 23") sealed off relative to oneanother; and in that the partial chamber (23') which on being subjectedto pressure brings about an unwinding of the tension band (21) from therotating drive shaft (19) or drive reel (22) is connected to thepressure space (17) and with the other partial space (23") is vented;and in that for the pressure-wise decoupling of the partial chambers(23', 23"), at least one sealing wall (29) is provided which extendsbetween the housing (24) and the periphery of the flexible draw band(21) which is wound onto the drive shaft (19), or onto the drive reel(22) concentrically mounted thereon wherein a region of the or eachsealing wall (19) facing the drive shaft (19), or drive reel (22), isformed so that it can pivot away in the direction of the pressure loadedpartial chamber (23) through the spring tension of the coiled up drawband regions and/or through an additional spring.
 18. Belt tensioner forsafety belt arrangements in motor vehicles having a holder (11) securedto the vehicle chassis and on the holder a belt winding reel (12) with asafety belt (13) wound thereon, a retraction spring arrangement (14,14') by means of with the safety belt (13) is continuously and lightlyso pretensioned in the retraction direction that it only lies loosely onthe vehicle occupant secured by it, a blocking arrangement between thebelt winding reel (12) and the holder (11) of the vehicle chassis whichblocks the safety belt (13) against being pulled out if suddenly pulled,and/or in the event of an acceleration resulting from an accident, andan additional drive (15) connected to the belt winding reel (12) via aclutch (20), wherein the additional drive (15) is normally separateddrivewise from the belt winding reel (12) by the disengaged clutch (20),but is abruptly activated in the event of a acceleration resulting froman accident and is rotationally fixedly coupled with the belt windingreel (12) via the then engaged clutch (20) in order, before the vehicleoccupant accelerates against the safety belt (13) to wind up such alength of the safety belt that the safety belt lies tightly on thevehicle occupant, with the additional drive (15) having a pyrotechnicalpressure source which is triggered or ignited by the accident dependentresponse of an acceleration sensor and which, on being triggered,abruptly builds up a high pressure in a pressure chamber (17), a torquegenerating device (18) which is energised by the pressure in thepressure chamber (17) and a drive shaft (19) connected to the torquegenerating device (18) which is rotationally fixedly connected with thebelt winding reel (12) via the clutch (20) characterised in that thetorque generating device (18) has at least one flexible tension band(21) which is secured at one end at a first mounting point (25) to theperiphery of the drive shaft (19), or to the drive reel (22) which isconcentrically mounted thereon, and is partially wound onto the driveshaft (19), or onto the drive reel (22), and is secured at the other endat a second mounting point (26) to the peripheral wall (30) of a bandrunning chamber (23) which is arranged within a housing (24) around thedrive shaft (19), or around the drive reel (22) and has a widthcorresponding to the width of the draw band (21) in such a way that thedraw band (21) extends through the band running chamber (23) whilesubdividing the band running chamber (23) into two partial chambers(23', 23") sealed off relative to one another; and in that the partialchamber (23') which on being subjected to pressure brings about anunwinding of the tension band (21) from the rotating drive shaft (19) ordrive reel (22) is connected to the pressure space (17) and with theother partial space (23') is vented; and in that for the pressure-wisedecoupling of the partial chambers (23', 23"), at least one sealing wall(29) is provided which extends between the housing (24) and theperiphery of the flexible draw band (21) which is wound onto the driveshaft (19), or onto the drive reel (22) concentrically mounted thereonwherein the retraction spring arrangement (14') is arranged at the sameend face of the belt winding reel (12) as the additional drive 115) andthe clutch (20), with the additional drive (15) and the clutch (20)lying between the retraction spring arrangement (14') and the beltwinding reel (12), and the parts of the additional drive (15) lying onthe axis of rotation (38) and the clutch (20) have axial cavities in theregion of the rotational axis for the passage of an inner shaft (41)which rotationally fixedly connects the retraction spring arrangement(14') with the belt winding reel (12).
 19. Belt tensioner for safetybelt arrangements in motor vehicles having a holder (11) secured to thevehicle chassis and on the holder a belt winding reel (12) with a safetybelt (13) wound thereon, a retraction spring arrangement (14, 14') bymeans of with the safety belt (13) is continuously and lightly sopretensioned in the retraction direction that it only lies loosely onthe vehicle occupant secured by it, a blocking arrangement between thebelt winding reel (12) and the holder (11) of the vehicle chassis whichblocks the safety belt (13) against being pulled out if suddenly pulled,and/or in the event of an acceleration resulting from an accident, andan additional drive (15) connected to the belt winding reel (12) via aclutch (20), wherein the additional drive (15) is normally separateddrivewise from the belt winding reel (12) by the disengaged clutch (20),but is abruptly activated in the event of a acceleration resulting froman accident and is rotationally fixedly coupled with the belt windingreel (12) via the then engaged clutch (20) in order, before the vehicleoccupant accelerates against the safety belt (13), to wind up such alength of the safety belt that the safety belt lies tightly on thevehicle occupant, with the additional drive (15) having a pyrotechnicalpressure source which is triggered or ignited by the accident dependentresponse of an acceleration sensor and which, on being triggered,abruptly builds up a high pressure in a pressure chamber (17), a torquegenerating device (18) which is energised by the pressure in thepressure chamber (17) and a drive shaft (19) connected to the torquegenerating device (18) which is rotationally fixedly connected with thebelt winding reel (12) via the clutch (20), characterised in that thetorque generating device (18) has at least one flexible tension band(21) which is secured at one end at a first mounting point (25) to theperiphery of the drive shaft (19), or to the drive reel (22) which isconcentrically mounted thereon, and is partially wound onto the driveshaft (19), or onto the drive reel (22), and is secured at the other endat a second mounting point (26) to the peripheral wall (30) of a bandrunning chamber (23) which is arranged within a housing (24) around thedrive shaft (19), or around the drive reel (22) and has a widthcorresponding to the width of the draw band (21) in such a way that thedraw band (21) extends through the band running chamber (23) whilesubdividing the band running chamber (23) into two partial chambers(23', 23") sealed off relative to one another; and in that the partialchamber (23') which on being subjected to pressure brings about anunwinding of the tension band (21) from the rotating drive shaft (19) ordrive reel (22) is connected to the pressure space (17) and with theother partial space (23") is vented; and in that for the pressure-wisedecoupling of the partial chambers (23', 23"), at least one sealing wall(29) is provided which extends between the housing (24) and theperiphery of the flexible draw band (21) which is wound onto the driveshaft (19), or onto the drive reel (22) concentrically mounted thereonwherein the clutch (20) can be engaged by the pressure generated by thetriggering of the pressure source (16), and in that the additional drive(15) is separated from the pressure source by a valve arrangement (59,64, 65) until the clutch (20) has engaged.
 20. Belt tensioner inaccordance with claim 19, characterised in that the clutch (20) has aslider (48) which is displaceable by the pressure of the pressure sourceand which brings about the engagement of the clutch (20) on displacementby the pressure of the pressure source.
 21. Belt tensioner in accordancewith claim 20, characterised in that the slider (48) only controls theengagement of the clutch (20) but that the engagement force is suppliedby a separate power source (44).
 22. Belt tensioner in accordance withclaim 19, characterised in that the clutch (20) has a first clutch partwhich is rotationally fixedly connected to the additional drive (15) orto the torque generating device (18), and a second clutch part which isrotationally fixedly connected to the belt winding reel (12) and isformed by the one guide disk (47) at the end face of the belt windingreel (12).
 23. Belt tensioner in accordance with claim 22, characterisedin that the first coupling part (44) is at least partially axiallydisplaceable in order to be able to engage with the counter-disk (47);and in that the first clutch part (44) can be brought into engagementwith the second clutch part (47) by a spring force but is held againstthe spring force until triggered.
 24. Belt tensioner in accordance withclaim 23, characterised in that the first clutch part is a clutch disk(44) which is biased by a spring force towards the counter-disk (47);and in that the clutch disk (44) is held at its outer periphery by abayonet closure connection (50, 57) in a disengaged position againstspring force; and in that the bayonet closure (50, 57) is releasable byrelative rotation of the clutch housing (58) and of the clutch disk(44).
 25. Belt tensioner in accordance with claim 24, characterised inthat the clutch disk (44) is itself of resilient design and consists inparticular of spring steel sheet and is stressed in the normal statebetween its center and its outer periphery to an essentially conicalshape so that the clutch (20) is disengaged and in that, on cancellationof the stressing through release by rotation of the bayonet closure (50,57), the clutch disk (44) is deformed as a result of its inherent springforce to its normal position, which is planar and thereby enables theengagement of the clutch
 20. 26. Belt tensioner in accordance with claim24, characterised in that the clutch disk (44) has engagement cut-outs(45) in the outer region with which axially projecting noses (46)arranged on the counter-disk (47) are associated.
 27. Belt tensioner inaccordance with claim 24, characterised in that a peripheral clearanceis present between the drive shaft (19), or the drive reel (22), and theclutch disk (44), such that, on rotation of the clutch disk (44)contrary to the belt retraction direction, the bayonet closure (50, 57)is first opened and then a further rotation of the clutch disk (44) ispossible until the engagement cut-outs (45) snap over the engagementnoses (46), in so far as this alignment was not already present onopening of the bayonet closure (50, 57).
 28. Belt tensioner inaccordance with claim 20, characterised in that the valve arrangement isformed by the slider (48) and a valve wall (59) on the slider (48) andalso by the inlet opening (68) of a pressure channel (61) leading to thepressure space (17).
 29. Belt tensioner in accordance with claim 28,characterised in that the valve wall (59) can be broken away from orpivoted away from the slider (48) by the pressure of the pressure source(16) as soon as the clutch (20) is engaged, and the valve wall (59) isdisplaced to the inlet opening (68) of the pressure channel.
 30. Belttensioner in accordance with claim 20, characterised in that the slider(48) is tangentially displaceably journalled in a displacement recess(49) which extends substantially tangentially to the clutch disk (44).31. Belt tensioner in accordance with claim 20, characterised in thatthe slider has a piston region (48') acted on by the pressure of thepressure source (16) and a preferably thinner actuating rod (48") at theside remote from the pressure actuation, with the actuating rod (48")cooperating via an abutment (62) which projects sideways on it with acounter-abutment (63) provided at the periphery of the clutch disk (44)in such a way that on advance of the actuating rod (48') in the event ofpressure loading of the piston region (48') the clutch disk (44) isturned through an angle such that it is rotated out of the engagedblocking position into a position in which the spring force can bringthe clutch disk (44) into engagement with the counter-disk, includingthe guide disk (47) of the belt winding reel (12).
 32. Belt tensioner inaccordance with claim 31, characterised in that, in a first stage of therotary movement of the clutch disk (44) brought about by the slider (48)the holding noses (50) at the periphery of the clutch disk (44) releasefrom the counter-noses (57) at the clutch housing (58) and, in a secondstage of the rotary movement--in so far as the engagement noses (46)have, in the first stage, not yet entered into engagement with theengagement cut-outs (45)--the clutch disk is turned further by theadvanced slider (48) until the engagement cut-outs (45) of the clutchdisk (44) have come axially into alignment with the engagement noses(46) of the counter-disk (47), whereupon the resilient clutch disk snapsover the engagement noses (46).
 33. Belt tensioner in accordance withclaim 32, characterised in that when triggered, the slider (48) engagesvia the abutments (62, 63) with the clutch disk (44) until the bayonetclosure (50, 57) has opened and the engagement recesses (45) and theengagement noses (46) have come into axial alignment; and in that, whenthis has taken place, the drive engagement between the slider (48) andthe clutch disk (44) is cancelled.
 34. Belt tensioner in accordance withclaim 31, characterised in that the rotary movement of the clutch disk(44) provided for the engagement of the clutch is opposite to therotation produced by the torque generating device; and in that aperipheral clearance is present between the drive shaft (19), or thedrive reel (22), and the clutch disk (44) which are rotationally fixedlycoupled on being energised by the torque generating device (18), withthe peripheral clearance permitting rotation of the clutch disk (44)contrary to the direction of rotation of the torque generating device(18) such that the retaining noses (50) of the clutch disk (44) come outof engagement with the counter-noses (57) at the housing and thereafter,through further rotation of the engagement cut-outs (45) of the clutchdisk (44) snap over the engagement noses (46) of the end guide disk (47)of the belt winding reel (12).